1. Cross-related Inventions
This invention is a continuation-in-part application of application Ser. No. 652,992, filed Sept. 21, 1984, which is a continuation application of application Ser. No. 360,501, filed Mar. 22, 1982 and a continuation-in-part application of application Ser. No. 740,682, filed June 3, 1985, which is a continuation-in-part application of application Ser. No. 652,992, filed Sept. 21, 1984.
2. Field of the Invention
This invention relates to driving sockets for screws and to punches for making such driving sockets.
3. Related Art
It is known (from for example U.S. Pat. No. 2,082,748, issued to Brown) to provide a combined driving socket that accepts either a square driver or a cross-point driver. In such a socket, the square recess is defined by four nearly vertical faces, and the cross recess by four wings disposed diagonally across the corners of the square, the wings each being defined by a pair of nearly vertical side walls and an end wall.
The term "vertical" applies only when the axis of the screw is itself vertical, but is used throughout the specification for clearer definition. The expression "nearly vertical" "or substantially vertical" means that the faces and walls in question have a draft angle of up to five degrees to permit easy withdrawal of a punch, or of a driver, from the socket, but apart from that the faces are, in substance, vertical.
It is known that the sharp corners of a square socket can give rise to crack-producing stress concentrations. The sharp corner is undesirable also because the extreme corner of the punch that has to form the sharp corner quickly becomes dulled and rounded due to the intense wear pressure at that point, and once the corner is no longer sharply formed, then the square driver cannot pass fully down into the square socket.
It can be inferred from the aforesaid U.S. patent that in order for substantially the whole length of the end wall to be clear of the square corner, the end wall must slope very steeply. The disadvantage here is that the cross-point driver is now not of the standard cross-point form called the Phillips form. Its blade edges slope more steeply, and it is blunter, as the patent shows. It could also be inferred that substantially the whole length of the end wall may be kept clear of the square corner if the end wall is positioned a long way from the axis of the screw. In this case, even though the angle of the end wall may be correct for the Phillips driver, the size of the Phillips driver that will be needed for the combination socket is enormous compared with the size of the standard square driver, or Robertson driver, that the socket needs.
In Canadian Patent No. 1,169,277 there is disclosed a combined socket for both Robertson and Phillips drivers. The socket has conventional vertical side faces forming a square for the Robertson driver. The faces are inclined so that, as is well known, they are engaged by a Robertson driver at the top. For the Phillips driver four wings are provided extending diagonally from the corners. The wings have nearly vertical and parallel side walls and a curved end wall. The curved end wall enables a standard size Phillips screw driver to be used while still enabling a Robertson driver to fully engage the socket.
The provision of curved end walls also means that the Phillips drive contacts these end walls below the top of the socket. However, the Robertson driver still contacts the socket at the top thereof. This has a number of disadvantages. It means that the stress resulting from the torque applied by the driver is greatest at the top, and this can effectively reduce the maximum torque that can be applied. It is necessary for the top of the vertical side faces to be formed accurately. But, in manufacture, the part of the punch forming the corners between the top face and the upper parts of the vertical side faces tends to wear quickly, necessitating frequent changes of the punch.
At the present time, a large number of different techniques are used for producing Robertson drivers and screw sockets. A common problem with all known techniques is the necessity to maintain fairly tight tolerances on items that are mass-produced. Conventional drivers are so dimensioned to have sides that are more steeply sloped than sides of corresponding sockets. As a result, when properly dimensioned, the driver will engage a socket near the top of the socket. This can produce a desirable cling effect, which helps maintain a screw on the driver, prior to inserting the screw in a socket.
However, by having different angles between the driver and the recessed sides, problems can arise, due to poor tolerances. One can have insufficient penetration of the driver into the socket, resulting in insufficient torque application and/or premature wear of the driver. Alternatively, the driver might engage fully to the bottom, resulting in loose fit or wobble.
Another problem in known arrangements is poor tolerances resulting in the driver primarily engaging a socket at the corners. This can result from wear of the punch giving a large radius to the socket corners, or build up in the corners of a plated screw. In either case, the driver will engage primarily at the corners causing undesirable stress concentrations.
It is desirable in a combined socket for the part accommodating the Robertson driver to engage between the top and the bottom of the socket. The, maximum stresses will occur at this point, and it should be possible to transmit a greater torque. Further, the socket design should be such that the torque is transmitted by abutting faces or at least along nominal line contacts, rather than point contacts. This avoids excessive stress concentrations. Further the socket design should be such that it permits large tolerances of places corresponding to areas of greatest punch wear. This increases punch life.